1. Field of the Invention
The present invention relates to a method for doping impurities, methods for producing a semiconductor device and an applied electronic apparatus. For example, the present invention is suitably applied to the production of a thin-film semiconductor element.
2. Description of the Related Art
Input/output devices become dramatically important with the progress of an advanced information age, and such devices are required to have greater functionality. Furthermore, in accordance with the remarkable widespread use of mobile devices in recent years, the development of a technique for forming a thin-film transistor (TFT) on a plastic substrate has been desired, the plastic substrate having advantages in that it is light, flexible, and nonfragile, compared with a general glass substrate. In such circumstances, the research and development of, for example, an active-matrix liquid crystal display (AMLCD) and a contact image sensor (CIS), each containing such TFTs, has been extensively conducted.
TFTs, each having a channel formed of a semiconductor film composed of silicon, are classified into two types based on a material constituting a layer through which carriers move (active layer): one type has a semiconductor film composed of amorphous silicon (a-Si); and another type has a semiconductor film composed of polycrystalline silicon (not monocrystalline but crystalline silicon) containing crystal phases. Polysilicon (poly-Si) and microcrystalline silicon (c-Si) are mainly known as the polycrystalline silicon.
Semiconductors composed of polycrystalline silicon, such as polysilicon and microcrystalline silicon, have advantages in that carrier mobility of such semiconductors is about 10 to 100 times as high as that of semiconductors composed of amorphous silicon, and thus have excellent properties as materials for switching elements. Furthermore, TFTs, each having an active layer composed of polycrystalline silicon, can operate at high speed and thus have been receiving attention recently as switching elements constituting, for example, various logic circuits such as domino logic circuits and CMOS transmission gate circuits; multiplexers, erasable programmable read-only memories (EPROMs), electrically erasable programmable read-only memories (EEPROMs), charge-coupled devices (CCDs), and random-access memories (RAMs), which contain those logic circuits; and driving circuits used for liquid crystal displays, electroluminescence displays and the like. In recent years, active-matrix liquid crystal displays employing TFTs as switching elements and as elements constituting peripheral driving circuits have been receiving attention, the TFTs each having a channel layer formed of a semiconductor film composed of such polycrystalline silicon. By constituting a TFT array composed of polycrystalline silicon semiconductor films, which can be formed at low temperature, on an inexpensive amorphous glass substrate, it is possible to produce a reflective panel display, such as a flat-screen television set, having a large area, a high definition, high quality, and a low cost.
In processes for producing polysilicon TFTs, the maximum temperature reaches about 1,000° C. Thus, insulating substrates used for the production of the polysilicon TFTs are composed of silica glass that has excellent heat resistance. That is, glass substrates that have relatively low melting points are hardly used for such processes. However, in order to achieve lower production costs for the liquid crystal displays, it is necessary to use glass substrates having low melting points. In recent years, the development of a low-temperature process in which the maximum temperature is 600° C. or less has proceeded and a device has actually been produced. Furthermore, the use of plastic substrates that can easily achieve larger area at lower temperatures has been studied. The deformation temperature of the plastic substrate is 200° C. at the highest even when the plastic substrate is composed of a heat-resistant material. Accordingly, when the plastic substrate is used, all processes must be performed at 200° C. or less, that is, at ultra-low temperatures compared with the case of a known art.
A trend toward large-area liquid crystal display requires a large-area semiconductor film. In low-temperature process for producing a polysilicon TFT, ion doping or plasma doping can be employed for injecting impurities into such a large-area semiconductor film with high throughput. The ion doping is performed as follows: An impurity gas is ionized and accelerated in an electric field without mass separation. Then a semiconductor film having a large area is irradiated with the accelerated impurity ions in a single operation. The plasma doping is performed as follows: An impurity gas and a gas for forming a film are simultaneously ionized, and then a film containing the impurity ions is formed the on a surface of a substrate. On the other hand, ion implantation is performed as follows: Impurity ions are subjected to mass separation, and an ion beam is produced. Then, a semiconductor film is irradiated with the ion beam. This ion implantation has the following disadvantages: The temperature of a substrate is increased because of the energy of the implanted ions; thus, the ion implantation cannot be applicable to a plastic substrate having a low melting point. In view of employing a beam scan and the size of apparatus, the ion implantation is not suitable for a large-area process. As described above, the ion doping and plasma doping have the advantages in their suitability for a trend toward large-area substrates. However, a semiconductor film subjected to ion doping or plasma doping contains a large amount of hydrogen. For example, when a plastic substrate is used, lower-temperature processes are required. However, the plastic substrate cannot be heated to a temperature required for removing hydrogen (400° C. to 460° C.). As a result, subsequent excimer laser annealing (ELA) for crystallization causes hydrogen in the film to blow out, thus damaging the semiconductor film. Furthermore, there is a problem in which the ion doping and plasma doping are not suitable for self-aligned processes in principle.
Laser-induced melting of predeposited impurity doping (LIMPID), which is disclosed in Japanese Unexamined Patent Application Publication Nos. 61-138131, 62-2531, 62-264619, and 9-293878, has recently been receiving attention as a process in which doping can be performed at ultra-low temperatures, i.e., 200° C. or less. The LIMPID is performed as follows: An impurity gas is ionized, and the resulting impurity ions are adsorbed on the surface of a semiconductor film. Then, the semiconductor film is melted by excimer laser irradiation, and thus the impurities move into the film. The film subjected to the LIMPID does not contain hydrogen. The LIMPID is suitable for not only a self-aligned process but also a low-temperature process. Consequently, the LIMPID has been receiving attention.
On the other hand, in recent years, the needs of the market for thin, light, large-sized, flat-screen television sets have been rapidly expanding instead of known television sets using a cathode-ray tube. In order to produce inexpensive large-sized flat-screen liquid crystal displays, efforts have been under way to improve throughput of their production lines. For example, a process for producing several display panels at a time from one substrate, the so-called “mother glass” has been developed.
As described above, the LIMPID using excimer laser is suitable for not only a self-aligned process but also a low-temperature process; therefore, the LIMPID has been receiving attention.
However, the LIMPID needs to use toxic impurity gases such as phosphine (PH3) and diborane (B2H6). Therefore, it is necessary to place a substrate in an evacuated chamber and then to perform plasma decomposition of the impurity gas, and to use a different chamber for each impurity gas in order to prevent cross-contamination. In addition, there are problems with contamination in the chambers and lines. Furthermore, the excessively introduced impurity gases during adsorption treatment increase the loads on the human body and the environment.
The trends toward large-sized flat-screen displays require glass substrates, called “mother glass”, each having a length of 1 m or more. The productions of such displays by known vacuum processes require larger vacuum chambers, larger transfer robots, larger production lines, and thus larger clean rooms. Those sizes reach a maximum. As a result, a capital investment entails a lot of costs.